A device or system having the ability to detect the presence, level, or quantity of particular materials, commonly referred to as a proximity detector, has many uses. For example, proximity detectors may be used to detect or sense the level of grain, aggregate, fluids or other materials in a storage container, or to detect the presence of a metal part on a production line. Proximity detectors have conventionally been produced in many forms including ultrasonic, capacitive, and Q sense detectors. Capacitive sensors, as shown in U.S. Pat. No. 4,345,167, sense the change in capacitance between two points (e.g., a sensing element and ground) using either a tuned oscillator or a timed RC delay circuit. Capacitive sensors are fundamentally sensitive to changes in the dielectric constant (permittivity) of the materials to be sensed. Q sense systems detect changes in the Q (merit factor) of a tuned circuit that includes the sensor element, as described in U.S. Pat. No. 5,832,772, and improvements thereto, as described in U.S. patent application Ser. No. 09/338,366. These systems are fundamentally sensitive to changes in both the permittivity and permeability of the materials to be sensed.
Conventional capacitive sensors are sensitive to changes in the dielectric constant of materials. These sensors typically include the material to be sensed as part of the dielectric material of a tuning capacitor. As the material to be sensed comes into spatial proximity of the capacitive sensor, the dielectric constant of the tuning capacitor changes, altering the capacitance of the tuning capacitor. The altered capacitance either changes the oscillation frequency of the tuned system or the time constant of an RC delay circuit. Either the oscillation frequency or the time constant is then compared to a nominal value (i.e., when the material to be sensed is not near the sensor) to determine the presence of the material.
An improved low-power proximity detector is a tuned sensor element. Using this technique, a monopole or dipole element is used as the sensor (i.e., antenna), which is coupled to circuitry designed to be sensitive to the Q of the tuned circuit. As the sensor comes near a lossy material (i.e., a material having a complex permittivity or complex permeability), the Q of the tuned circuit decreases. This decrease in Q can be detected to determine if the material is in proximity to the sensor.
Both of these types of sensors are sensitive to the quantity of electromagnetic field lines that intersect the material to be sensed. For this reason, it is important that a significant quantity of the material be in close proximity to the sensing element. Most proximity detectors are packaged into a cylindrical shape, with the sensing element at one end of the cylinder and the wiring connections located at the opposite end. This conventional configuration has a significant disadvantage when mounted vertically and used with materials that have a significant angle of repose when stored in a container (e.g., animal feed or grain). As shown in FIG. 1, when the material 20 naturally falls at a given angle, and does not fill in underneath the sensor, there is a void immediately in front of the sensor element at the tip of the sensor package. Moving the sensor element to the corner of the housing is disadvantageous because it results in a system that is sensitive to rotation.
A need remains for a proximity detector housed in a package designed to improve the responsiveness of the sensor to materials that do not flow freely (i.e., naturally rest with a significant angle of repose). The improved proximity detector would preferably ease the effort necessary to accurately position the sensing element relative to the material to be sensed.